1. Field of the Invention
The present invention relates to a transparent conductive film and a method for producing the same, and more specifically, the present invention relates to a transparent conductive film composed of zinc oxide (ZnO) as a main component and a method for producing the same.
2. Description of the Related Art
In recent year, transparent electrodes have been widely used for flat-panel displays and solar cells. As a material for the transparent electrodes, ITO (indium tin oxide) has been widely used.
However, indium is expensive and there are fears of exhaustion of resources, and thus transparent electrodes using other materials have been increasingly required. As transparent electrodes not using In, the development of ZnO-based transparent electrodes using zinc (Zn) oxide (ZnO) which is low-priced and can be stably supplied has been advanced.
Although ZnO with a stoichiometric composition is an insulator, conductivity can be imparted by excess electrons due to oxygen defects or element replacement (doping) at a Zn site. As such transparent electrodes using ZnO as a main component, transparent electrodes having a resistivity ρ on the order of 10−4 Ωcm can be produced under the present conditions.
However, ZnO-based transparent conductive films have the problem of insufficient moisture resistance from the viewpoint of practical application. Namely, conventional ZnO-based transparent conductive films contain many oxygen defects and thus have the problem of decreasing the amount of carriers by moisture adsorption (re-oxidation) on the oxygen defects to increase resistance when allowed to stand in a high-humidity environment. As a criteria for moisture resistance of transparent electrodes using ITO, a rate of resistance change after the passage of 720 hours in an atmosphere at 85° C. and 85% RH is ±10%. However, ZnO-based transparent conductive films satisfying this requirement have not yet been obtained.
Furthermore, when a ZnO-based transparent conductive film is formed on a flexible substrate, which is predicted to be widely used in the future, there is the problem of increasing the deterioration in the transparent conductive film by the influence of moisture permeated from the surface of the transparent conductive film and moisture permeated through the flexible substrate because the flexible substrate is permeable to moisture.
In order to resolve the problems, various methods for improving the moisture resistance of ZnO-based transparent conductive films have been investigated. These methods are roughly divided into the following two methods:
(1) A method of providing a SiN barrier layer for suppressing moisture permeation from the substrate side.
(2) A method of improving the quality (crystallinity) of a ZnO film by heating film formation.
However, a ZnO-based transparent conductive film having practicable moisture resistance has not yet been obtained.
Examples of a technique for imparting conductivity by doping ZnO with an element include the following:
(a) A method of doping a ZnO film with an impurity using a molecular beam of any one of the atoms in the group IA (H), the group IIIA (B, Al, Ga, and In), and the group VII (F, Cl, I, and Br) in forming the ZnO film using a ZnO molecular beam or Zn and O molecular beams, for decreasing the electric resistance with high controllability (refer to Japanese Unexamined Patent Application Publication No. 7-106615).
(b) A transparent conductor including a transparent conductive film laminated on a substrate and composed of zinc oxide doped with a group VB or VIB element in the periodic table, the content of the element being 0.1 to 10 atomic % relative to the total number of element atoms and zinc atoms (refer to Japanese Unexamined Patent Application Publication No. 8-050815).
(c) An organic EL element including an anode, a cathode, and an organic layer sandwiched between the two electrodes, the anode including a transparent conductive film composed of a material containing at least one of the oxides of Ir, Mo, Mn, Nb, Os, Re, Ru, Rh, Cr, Fe, Pt, Ti, W, and V (refer to Japanese Unexamined Patent Application Publication No. 11-067459).
(d) A transistor including a transparent conductive material such as conductive ZnO doped or not doped with any one the group II, VII, I, and V elements (refer to Japanese Unexamined Patent Application Publication No. 2000-150900).
(e) A transparent conductive film including a zinc oxide thin film having a c-axis/a-axis orientation ratio of 100:1 or more, the zinc oxide film being doped with at least one of group III and VII compounds of aluminum, gallium, and boron (refer to Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-276943).
(f) An indium-zinc oxide hexagonal layered compound represented by the general formula, (ZnO)m.In2O3 (m=2 to 20) wherein an In or Zn element is replaced by at least one element selected from the group consisting of Sn, Y, Ho, Pb, Bi, Li, Al, Ga, Sb, Si, Cd, Mg, Co, Ni, Zr, Hf, Sc, Yb, Lu, Fe, Nb, Ta, W, Te, Au, Pt, and Ge, the layered compound having an average thickness of 0.001 μm to 0.3 μm and an average aspect ratio (average long diameter/average thickness) of 3 to 1000 (refer to International Application Publication No. 2001/056927 pamphlet).
These ZnO-based transparent conductive films have the above-described problem of moisture resistance.